The present invention relates to Wideband Code Division Multiple Access (WCDMA) technology. More particularly, and not by way of limitation, the present invention is directed to a system and method for improving the reliability for the transmission of Multiple-Input-Multiple-Output (MIMO) channel quality indicators (CQI) and antenna weight indicators (AWI).
With the introduction of Multiple-input-multiple-output (MIMO) transmission technology to increase spectral efficiency and system throughput of WCDMA systems, more detailed reporting of channel quality indicators (CQI) in support of MIMO transmission is needed. For example, for 2 by 2 MIMO, 2 CQI numbers, each for a data stream, are needed. According to W-CDMA Release 7, each of these CQI numbers is represented by 4 bits, and thus overall 8 bits are used for CQI feedback. In addition, a user terminal (UE) needs to feed back a 2-bit antenna weight indicator (AWI). Thus the total UE feedback is 10 bits.
In Release 6 of WCDMA systems (3GGPP TX 25.212, v.6, “Multiplexing and Channel Coding (FDD) (Release 6)”), 32 different types of CQI messages can be represented by 5 information bits. With a spreading factor of 256, these information bits can be encoded into 20 channel coded bits in 2 slots. Such a channel is built upon the first order Reed-Muller code. The minimum distance of the Release-6 CQI code has minimum Hamming distance 8. In the below discussion, (n,k,d) is used to refer to a class of block codes that encodes k information bits to produce a codeword of n-bits long and has a Hamming distance between any pair of distinct codewords no less than d. Thus, the Release 6 code for CQI is a (20,5,8) code. Sometimes, the notation (n,k) is used to describe the length of the codeword and the input information block.
As mentioned earlier, 10 CQI/AWI feedback bits are needed to support 2 by 2 MIMO operations. Providing channel coding protection for the lengthened feedback message is proposed in “Definition of HS-DPCCH coding for FDD MIMO operation in Rel-7” 3GPP TSG RANI Tdoc R1-063422, Meeting #47bis, November 2006; based on a (20,10) code shortening of the 2nd order Reed-Muller. The generator matrix of this code is
                              G                      20            ,            10            ,            4                          =                                            1                                      0                                      1                                      0                                      1                                      0                                      1                                      0                                      1                                      0                                      1                                      0                                      1                                      0                                      1                                      0                                      0                                      0                                      0                                      0                                                          0                     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        0                                      0                                      1                                      0                                      1                                      0                                      1                                      0                                      1                                      0                                      0                                      0                                      0                                      0                                                          0                                      0                                      0                                      0                                      0                                      1                                      1                                      0                                      0                                      0                                      0                                    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                                   0                                      0                                      0                                                          (        1        )            For any linear block code, one can calculate its distance spectrum by finding the distance distribution of all the codewords to the all-zero codeword (zero in all the positions). For example, the distance spectrum of the above code is given by                0 1        4 57        6 120        8 262        10 144        12 262        14 120        16 57        20 1.This means that among all the codewords, there is one codeword (the all-zero codeword itself, [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]) having zero Hamming distance from the all-zero codeword, there are 57 codewords having Hamming distance 4 from the all-zero codeword, there are 120 codewords having Hamming distance 6 from the all-zero codeword, there are 262 codewords having Hamming distance 8 from the all-zero codeword, there are 144 codewords having Hamming distance 10 from the all-zero codeword, there are 262 codewords having Hamming distance 12 from the all-zero codeword, there are 120 codewords having Hamming distance 14 from the all-zero codeword, there are 57 code-words having Hamming distance 16 from the all-zero codeword, and there is 1 codeword having Hamming distance 20 from the all-zero codeword. Thus, the minimum Hamming distance between any distinct codewords in the above code is 4, making this a (20,10,4) code.        
In general, there are numerous codebooks of (n,k,d) code having the same distance spectrum. For example, any permutation (shuffling the transmission order) on a (n,k,d) code results in another (n,k,d) code having the same distance spectrum. Also, any common masking m applied to all the codewords of a (n,k,d) code results in another (n,k,d) code having the same distance spectrum. We will use a simple example below to illustrate this. Consider a simple generator matrix
      G    =          [                                    1                                1                                0                                0                                0                                0                                1                                1                              ]        ;The four codewords generated by this generator matrix are
            0              0              0              0                  1              1              0              0                  0              0              1              1                  1              1              1              1.      The distance spectrum of this codeword is
0 1
2 2
4 1.
Thus, one codeword at Hamming distance 0 away from the all-zero codeword, two codewords at Hamming distance 2 away from the all-zero codeword, and one codeword at Hamming distance 4 away from the all-zero codeword. Now, changing the order of the 2nd and 3rd encoded bit in the above code, the four new codewords are
0 0 0 0
1 0 1 0
0 1 0 1
1 1 1 1.
It is easy to see that the distance spectrum remains the same. Now further perform masking on the above code using a common mask of [1 1 1 0], we have
            0000      +      1110        =    1110              1010      +      1110        =    0100              0101      +      1110        =    1011              1111      +      1110        =    0001.  It is easy to see that the new code preserves a distance spectrum of
0 1
2 2
4 1.
The minimum Hamming distance of the above proposed (20,10) code is only 4. This minimum distance implies a significantly weaker protection for CQI information bits compared to the existing code in Release 6 for CQI protection. A comparison of the Rel6 CQI channel code and the above proposed code based on the generator matrix G20,10,4 shown above is provided in the graph shown in FIG. 4 (comparing Rel6 channel code (20,5,8) and a (20,10,4) code for MIMO CQI). It can be seen that, with the above proposed code, one more dB is required to carry each of the CQI information bits. At the same time, the number of information bits to carry has doubled. In summary, significantly more power is needed to transmit the detailed MIMO CQI report reliably.
It would be advantageous to have a system and method for providing CQI and AWI information to a base station that overcomes the disadvantages of the prior art. The present invention provides such a system and method.